Generation XXL - why ARE we all fatter than our mothers?

Model Yasmin Le Bon, 44, posed happily at a charity event earlier this month with her three daughters aged between 14 and 19.

They are clearly a happy, healthy, good-looking family but to be brutally honest, one of the most striking things about the image is that two of the girls are noticeably larger than their svelte mother.

This might just be one of those things, a genetic issue perhaps - after all, their pop star father, Simon, has been known to struggle with his tendency to paunchiness. But they are not alone in out-sizing their mothers.

Take the Geldof girls, Fifi, Peaches and Pixie. They vary in size, but are all curvier than their late mother, Paula Yates, who in her 20s boasted a tiny 22in waist.

Demi Moore, 46, looks tiny compared with her strapping daughters, and, of course, there is always Chastity (or should that be Chas?) Bono, whose vast bulk dwarfs her mother's perennially slim figure, even though Cher is now 63.

And despite Fergie's well-documented struggles with her weight, at 49 she is still slimmer with better legs than her daughters, Beatrice and Eugenie. So what's going on?

It always used to be considered normal for women to be slender in their youth, before succumbing to middle-aged spread. Yet increasingly, pictures of celebrity mothers with their teenage or adult offspring reveal that this is no longer the case.

A study by Size UK, sponsored by the Department for Business, Innovation and Skills, revealed that women's waists are now 6in bigger than they were in 1951, and while size 12 used to be the average size, now it is a size 16.

'We were shocked by the results,' said Philip Treleaven, director of the UK National Sizing Survey.

'We expected an increase, but the waistline has just exploded.'

And it's not just our waists that are increasing. Around 40 per cent of women are overweight or obese.

The average bra size in the UK is now 36C, whereas a decade ago it was 34B. Marks & Spencer now stocks bras in a massive J cup size.

All this is not such a surprise to me. I remember my own teens and 20s in the late Seventies and early Eighties. My peers were nearly all slim or even skinny.

We had very little money and wouldn't have dreamed of wasting what we had on snacks and burgers.

But I am constantly astonished by the number of girls in their teens and early 20s shamelessly displaying vast muffin tops spilling over the tops of their jeans, flabby bellies escaping from short tops and bulging thighs emerging from too-short skirts.

They clearly have plenty of money for junk food. I may sound judgmental, but I have to confess that I, too - in my 40s and a size 12 - am far less willowy than my size 8-10 mother, even though she is in her 60s.

It doesn't help that she does daily dance classes and never stops running around, while I work sitting at a desk. But, if I'm honest, there is more to it than that.

Weighty issue: Writer Leah Hardy (right) and her slimmer mother

She eats to live, hardly ever drinks, and, I'm sure, thinks going to a restaurant is a dreadful waste of money.

She is astonished by how much younger women eat and would never dream of scoffing, say, an entire pizza.

In short, she has self-discipline. I suspect that self-control is one of the biggest reasons why more and more mothers are staying slimmer than their daughters. Tam Fry of the National Obesity Forum agrees.

'We have a generation of people who are heavier than the previous generation, and it's all down to over-eating and under-exercising. This generation is more sedentary than their mothers.'

Obesity really started to escalate in 1989, when microwaves and other gadgets became commonplace, fast-food restaurants boomed and we started eating junk food.

The most worrying thing about the growing size of young women is the implication it has for future generations.

A new study from Plymouth University shows that an obese mother is ten times more likely to have an obese daughter, and that this is not down to genetics, but daughters copying their mothers' bad habits.

Worryingly, many women don't even seem to notice that their offspring are getting tubby.

Tam Fry says: 'The evidence shows that parents are very bad at noticing their children are overweight. In fact, many parents of fat children are in denial.'

Also, fat is catching. The fatter their peers, the more likely it is that even slim young women will gain weight by emulating them. He also warns that we must not dismiss plump teenagers as having temporary 'puppy fat'.

'Hormones do cause girls to gain weight at puberty, and that's normal, but if they get too fat at that age it is unlikely to disappear.'

I have a daughter who is only four. Right now she's bursting with energy and is as lean as a whippet, but I do worry that she might join the legion of fat teens of the future.

So though keeping slim is not easy for me, I believe it is my job to model healthy self-discipline around food, to shift the pounds when I gain them, choose apples over cakes and encourage a healthy, active lifestyle.

And if, in her teens or 20s, my daughter gets fatter than her middle-aged mother, I shall feel that I have failed her.